The Mayan calendar used 3114 BC as their reference.More recently is the radiocarbon date of 1950 AD or before present, BP.How It Works: Carbon has 3 isotopic forms: Carbon-12, Carbon-13, and Carbon-14.
Radiocarbon is then taken in by plants through photosynthesis, and these plants in turn are consumed by all the organisms on the planet.
So every living thing has a certain amount of radiocarbon within them.
After an organism dies, the radiocarbon decreases through a regular pattern of decay. The time taken for half of the atoms of a radioactive isotope to decay in Carbon-14’s case is about 5730 years.
Half-lives vary according to the isotope, for example, Uranium-238 has a half-life of 4500 million years where as Nitrogen-17 has a half-life of 4.173 seconds!
In a stratigraphical context objects closer to the surface are more recent in time relative to items deeper in the ground.